In recent years, there has been an increasing demand for high performances in a silver halide photographic light-sensitive material, and further improvements are strongly demanded toward much higher photographic properties, particularly toward higher sensitivities, lower fog densities and higher image qualities.
In association with such tendencies towards higher sensitivities and higher image qualities, improvements towards higher pressure resistances are also demanded of a silver halide photographic light-sensitive material.
In general, a silver halide photographic light-sensitive material (hereinafter occasionally abbreviated as a light-sensitive material) is subjected to various types of pressure, for example, mechanical stresses such as bending and rubbing applied to a light-sensitive material in the processes of manufacturing, cutting and processing as well as at the time of loading it into a cartridge or a camera.
As is well known, application of such stresses causes pressure fogging (also termed pressure sensitization) or lowering of sensitivity (pressure desensitization) of a light-sensitive material; accordingly, there is a strong demand for a light-sensitive material which is immune from deterioration in photographic properties when subjected to pressure.
Among various methods which have so far been proposed as measures for improving the pressure resistance, methods which improve the pressure resistance of silver halide grains themselves are regarded to be more practical and more effective than methods which alleviate external pressure exerted on silver halide grains by use of a polymer or a plasticizer.
Accordingly, there has actively been studied in recent years the use of emulsions comprising core/shell-type silver halide grains having a silver iodobromide phase of high silver iodide content in the core. For example, Japanese Pat. O.P.I. Pub. Nos. 99433/1984, 35726/1985 and 147727/1985 disclose color negative films improved in pressure resistance by use of core/shell-type silver iodobromide grains having, inside of the grains, a high silver iodide content phase comprising 10 mol % or more silver iodide.
In the preparation of such core/shell-type silver halide grains, the formation of high iodide content cores needs to increase the growth rate of silver halide grains by raising the solubility of silver halide during the growth to compensate the low solubility of silver iodide.
As typical means to raise the solubility, there are known elevating the temperature during the mixing process, adding a silver halide solvent, and raising the pH and pAg during the mixing process.
However, core/shell-type emulsions have problems that silver halide grains are liable to aggregate in the process of manufacturing emulsions during which iodides necessary to form high iodide content cores are added to a reaction system, and that emulsions after grains are grown are apt to fog and deteriorate in graininess.
Further, Japanese Pat. O.P.I. Pub. NO. 943/1990 discloses a technique to obtain an emulsion of high sensitivity and low pressure fogging property by continuously changing the iodide content inside of grains, without giving a clear core/shell structure to grains. However, this technique has a disadvantage of greatly deteriorating pressure desensitizing resistance, though it brings about improvements in pressure fogging.
As stated above, it is not easy to develop a light-sensitive material having a high sensitivity, a high resolution and improved resistances to pressure fogging and pressure desensitizing and, therefore, a new technique has been strongly sought for the development of such a light-sensitive material.